1. Field of the Invention
The present invention relates generally to a method and apparatus for increasing the overall efficiency of air conditioning systems by the introduction of a liquid refrigerant into the discharge of a single or multiple stage compressor. In one aspect of the invention, desuperheating of compressed discharge vapors is achieved by the evaporative introduction of a liquid refrigerant between multiple compression stages of an air conditioning or refrigeration system, where this refrigerant has a high latent heat of vaporization. Alternatively, desuperheating of compressor discharge vapors is achieved by the recycle of liquid refrigerant to the discharge of a single or multiple stage compressor.
2. Description of the Prior Art
Air conditioning and refrigeration systems are major consumers of power in both the U.S. and abroad. For example, it has been estimated that in the United States alone there are some 28,000 grocery outlets which annually consume some 1 million kWh of electricity. If such systems could be made only ten percent more efficient, the savings in electricity would translate into annual domestic savings of $140 million (at 5.cent./kWh) or about five million barrels of oil.
In the normal operation of a refrigeration or air conditioning system, low pressure liquid refrigerant is evaporated to achieve a low-pressure vapor. The latent heat of vaporization required for this phase change produces the resultant refrigeration effect. These low pressure vapors are then compressed to a high-pressure, superheated state, where they then enter a high-pressure heat exchanger where energy is removed. In operation, the first section of the high-pressure heat exchanger functions as a desuperheater, while the latter section functions as a condenser. The condensed liquid from the condenser is then throttled through an expansion valve and is returned to the evaporator.
Functionally, a desuperheater is relatively space inefficient, since while the desuperheater removes only a small fraction of the energy from these compressed superheated vapors, the desuperheater often occupies a relatively large fraction of the overall high-pressure heat exchanger (i.e., desuperheater and condenser) area. This inefficiency results because the desuperheater has a low internal heat transfer coefficient due to the presence of a vapor film created during the normal operation of such a system. In comparison, the condenser has a relatively high internal heat transfer coefficient. Clearly then, when the entire high-pressure heat exchanger functions as a condenser, the increased condenser area lowers both the condenser temperature and pressure, thus resulting in a reduction of overall compressor work.
Since more energy is required to compress hot vapors than cool vapors, energy costs may thus be reduced by desuperheating superheated vapors produced during the compression process. Known in the art are devices designed to lower the temperature of the compressed vapors by the introduction of a liquid refrigerant to the exterior of a closed compression system. One such device is seen in U.S. Pat. No. 4,242,875 - Brinkerhoff. This patent describes an isothermal piston compressor apparatus wherein a compression chamber and a spray injection heat exchanger are placed in a heat exchange relationship to each other. More specifically in this patent, heat exchange coils from a closed compression chamber extend up into an evaporation chamber so that the gases flowing through these coils may be cooled prior to recompression.
Disadvantages of this concept include the undesired addition of "dead space" to the total compression system. The additional volume created by this coil may not be effectively "swept" by the compression piston, thus resulting in an overall lowering of system pressure and volumetric efficiency. Additional problems associated with this concept include the difficulty in exchanging heat between the compressed vapors and the evaporating liquid. In this, the external evaporation temperature must be substantially lower than the temperature of the compressor. This extreme heat gradient places an additional load on the compressor which attempts to purge the evaporation chamber.
The introduction of liquid directly into the compression chamber of refrigeration systems is also well-known in the art. Previous efforts in this area have described the spray introduction of liquid into the compressor chamber in a manner analogous to a fuelinjected automobile engine. Compressor systems including means for injecting liquid refrigerant directly into the compressor for mixture with the vapors being compressed therein are described for example in U.S. Pat. Nos. 3,109,297 - Rinehart and 3,105,633 - Dellario. In such compressor systems, liquid refrigerant from the condenser is introduced into the compression chamber through an injector port when the gas pressure in the compression chamber is lower than the pressure of the condenser. The injected liquid refrigerant vaporizes thereby cooling the discharge gases sufficiently to provide the desired cooling of the system motor by the discharged vapors.
A variety of other methods have also been pursued in order to provide lubrication, sealing and cooling of the system compressor. Such a system is seen for example in U.S. Pat. No. 3,105,630 Lowler et al. - wherein an oil or other suitable liquid is injected in the compression chamber of the compressor for the purpose of cooling, lubricating and sealing the internal parts of the compressor. Liquid recycle directly to the compression chamber is also described in U.S. Pat. No. 2,404,660 - Rouleau. This invention relates to a piston type compressor where an atomized liquid is delivered to the cylinder during that portion of the cylinder stroke in which compression heat is being generated, this liquid then being vaporized during compression.
The primary motivations for liquid recycle, have been to cool electric compression motors, prevent overheating of the compressor itself, and provide lubrication and sealing. The use of liquid recycle, however, generally provides an adverse effect on system efficiency if refrigerants with a low latent heat of vaporization (such as chlorofluorocarbons) are employed. Other disadvantages associated with this and similar designs include the possibility of "slugging" unvaporized refrigerant liquid, which often results in damage to the system compressor. Further, the short residence time in high-speed compressors makes it difficult to vaporize a significant amount of the liquid and achieve the desired cooling benefits. Although direct injection of the refrigerant liquid into the compressor achieves a maximum reduction in energy, direct injection is exceptionally difficult to implement in a practical manner.
Multistage compression with evaporative intercooling of the interstage vapors by saturation with recycle liquid can approach the performance of a direct injection system by infinitely increasing the number of compression stages. Further, multistage compression with evaporative intercooling can be adapted to any type of rotary, screw, scroll, centrifugal or piston compressor. However, many types of compressors, centrifugal compressors in particular, may be damaged by the introductions of a liquid refrigerant directly into the compressor intake. Therefore, for these and similar types of compressors, direct injection systems are not practical.
An evaporative intercooler using a liquid reservoir has also been described in the art. In his book "Refrigeration and Air Conditioning" (1958), Stoecker describes an evaporative intercooler where a tank filled with liquid refrigerant is placed between the compression stages, wherein superheated vapors passing through the liquid become saturated. This technique enhances energy efficiency for ammonia but has a detrimental energy efficiency effect for Refrigerant 12 (dichlorodifluoromethane). Further disadvantages associated with this technique include both the required space and overall capital costs, since in this system the tank diameter must be sufficiently large to ensure a vital disentrainment of liquid.